Patent Literature 1 discloses that a fresh water generation system which performs the steps of: filtering microorganism treatment effluent produced from the waste water which contains organic substances and is to be microbiologically treated, via a primary semi-permeable membrane; mixing the resultant concentrated water into seawater; and filtering the mixed water via a secondary semi-permeable membrane. Those steps allow an osmotic pressure of seawater to be reduced, resulting in the power saving of pump supply needed for the filtration step of the secondary-semi permeable membrane.
According to such a fresh water generation system, when a pressure loss of a semi permeable membrane apparatus having primary and secondary semi-permeable membranes reaches a predetermined value, the respective semi-permeable membranes (or reverse osmotic membranes) are chemically cleaned so as to remove microorganisms treating waste water, the microorganisms adhering to the semi-permeable membranes. Herein, the pressure loss of the semi-permeable membrane apparatus represents a value calculated by subtracting a pressure of the concentrated water from a pressure of the injection water in the semi-permeable membrane apparatus.
In the meantime, even if a semi-permeable membrane is chemically cleaned, multiple times repeated chemical cleaning gradually increases a pressure loss of the semi-permeable membrane apparatus at the initial operation period just after each chemical cleaning. This requires replacement of a semi-permeable membrane. Thus, the replacement timing of a semi-permeable membrane is determined, for example, at the time when a pressure loss of the apparatus reaches about 3-fold compared to the initial pressure loss thereof.
Alternatively, a low biofouling semi-permeable membrane which prevents microorganisms of treating effluent from adhering to the membrane thereof has been developed recently. Herein, such a low biofouling semi-permeable membrane (for example, TORAY INDUSTRIES, INC.; TML 20 TYPE) is used for a primary semi-permeable membrane (or a semi-permeable membrane in an apparatus of treating waste water), which filters microorganism treating effluent in which microorganisms are easily cultivated; the microorganism treating effluent containing a lot of organic substances compared to seawater.